1. Field of the Invention
The present invention relates generally to communication systems, and in particular, to a method for controlling the gain of a transmission signal in a communication system.
2. Description of the Related Art
A communication system, such as a cordless telephone has a microphone for receiving a voice input (hereinafter xe2x80x9ctransmission signalxe2x80x9d) from a communicating or sending party. The microphone outputs the transmission signal to a receiving party. On the sending side of the communication link, the transmission signal is unmuted during a conversation (or call) state and the transmission signal is muted during an idle state. As a result, the transmission signal is sent to a receiving party only during a conversation or call state.
The transmission signal is switched from a mute state to an unmute state at the moment the communication system switches from the idle state to the call state. Likewise, the transmission signal switches from the unmute state to the mute state at the moment the communication system is switched from the call state to the idle state. As such, the transmission signal is abruptly changed, as it undergoes a state change (i.e., switch from the mute state to the unmute state and from unmute to mute). The abrupt change of state causes undesirable noise spikes in the transmission signal which deteriorate the speech quality. The noise spikes are generated primarily at an input stage of the microphone, composed of an operational amplifier.
FIG. 1 illustrates changes in a gain of the transmission signal in a conventional cordless telephone and FIG. 2 shows the corresponding operational state of the transmission signal. As illustrated in FIG. 1, the cordless telephone changes operational states in the following sequence: idle statexe2x86x92call statexe2x86x92idle state. In the idle state, the cordless telephone minimizes the gain of the transmission signal so as to mute the transmission signal. While in the idle state, when a call is initiated, the gain of the transmission signal abruptly increases so as to unmute the transmission signal. Further, when the cordless telephone re-enters the idle state after completion of the call, the gain of the transmission signal is abruptly decreased minimizing gain so as to mute the transmission signal. As described above, the gain of the transmission signal is abruptly switched when the call is both initiated and terminated. As previously stated, the abrupt switching of the transmission signal gain introduces noise spikes in the transmitter, as shown in FIG. 2. The noise spike cause a deterioration in speech quality, thereby reducing the reliability of the communication system. As such, there exists a need for a method of controlling the gain of a transmission signal to prevent noise spikes when a call is both initiated and terminated.
It is therefore an object of the present invention to provide a method for controlling a gain of a transmission signal to prevent noise spikes from being generated when a call is initiated and terminated.
According to the present invention, there is provided a method for gradually increasing the gain of an amplifier for amplifying a transmission signal (i.e., microphone output) to reach an unmute state, when a call is initiated comprising the steps of: determining when a call is initiated; increasing the gain of an amplifier in discrete steps to reach an unmute state; determining when the call is terminated; and decreasing the gain of the amplifier in discrete steps to reach a mute state upon termination of said call.
According to another aspect of the invention, there is provided a method for gradually decreasing the gain of an amplifier to reach a mute state when a call is terminated comprising the steps of: determining when a call is initiated; increasing the gain of a mute circuit for receiving a signal output from a microphone to reach an unmute state; determining when a call is terminated; and decreasing the gain of the mute circuit in discrete steps to reach a mute state upon termination of said call.